The present invention relates to the use of induction heating systems, more particularly, to the use of smart susceptors to selectively heat a part or parts during a manufacturing process.
Generally, induction heating processes may be carried out using any material that is electrically conductive and that generates heat when exposed to an electromagnetic flux field. Often, induction heating is used to directly heat an electrically conductive part during a manufacturing process. The electromagnetic flux field can be generated by an electromagnetic coil that surrounds the part and is supplied with alternating, or oscillating, electrical current from a power source. However, when a simple electromagnetic coil design and thorough heating of the part are desired, the induction heating process typically requires the use of a susceptor that encapsulates the part. Susceptors are not only electrically conductive, but also have a high thermal conductivity for a more efficient and thorough heating of the part. Therefore, manufacturing processes requiring localized heating, relatively quick heat-up and cool-down times, a more efficient use of power, or customized thermal properties that enable fabrication, benefit from induction heating processes that use susceptors.
Certain manufacturing processes require heating up to, but not beyond, a certain temperature. A select type of susceptor, often referred to as a xe2x80x9csmart susceptor,xe2x80x9d is constructed of a material, or materials, that generate heat efficiently until reaching a threshold, or Curie, temperature. As portions of the smart susceptor reach the Curie temperature, the magnetic permeability of those portions drops precipitously. The drop in magnetic permeability has two effects, it limits the generation of heat by those portions at the Curie temperature, and it shifts the magnetic flux to the lower temperature portions causing those portions below the Curie temperature to more quickly heat up to the Curie temperature.
Mechanical part manufacturing processes often require the controlled application of heat, such as when consolidating composite panels, or for metal forming processes such as brazing and superplastic forming. To this end, smart susceptors have been employed in combination with dies for mechanical forming such as the invention described in U.S. Pat. No. 5,728,309 to Matsen et al. commonly assigned and incorporated herein by reference. Matsen discloses an induction heating workcell 10 that includes a pair of ceramic dies 20, 22 mounted within a pair of strongbacks 24, 26. A pair of cavities 42, 44 defined by the dies hold respective ones of a pair of tool inserts 46, 48. A retort 60 is positioned between the tool inserts and includes a pair of susceptor sheets sandwiching a pair of metal, or composite, part panels. The tool inserts define a contoured forming surface 58 that has a shape corresponding to the desired shape of the upper and lower mold line surfaces of the completed part. An induction coil 35 is embedded into the dies and surrounds the cavities, tool inserts and the retort.
Suction pressure can be used to hold the susceptor halves to the dies when handling the dies before the start of the process. During the process, the retort is heated to forming or consolidation temperature by energizing the induction coil which generates an electromagnetic flux field. The flux field causes the susceptor plates to generate heat, while the dies and tool inserts have a relatively low magnetic permeability and therefore generate little heat. Internal tooling pressure is used to hold the susceptors against the dies during processing. This pressure is either supplied by sealing around the perimeter of the dies or using pressurized bladders. The application of heat and pressure is continued until the metal part panels are properly brazed, or formed, or the resin in the composite panels is properly distributed to form the completed part.
Advantageously, the susceptor may be custom tailored to the desired thermal leveling temperature by using different alloy materials such as cobalt/iron, nickel/iron, iron/silicon, or amorphous or crystalline magnetic alloys. Also, the susceptor can be designed to have several different thermal leveling temperatures by using multiple layers of different alloys that are tuned to different Curie temperatures. Control of the thermal processing temperature at the thermal leveling point, however, is also important because the processing temperature about the leveling point may vary as much as xc2x110xc2x0 F. Supplying too much power results in an overshoot of the desired processing temperature, while supplying too little power results in a long wait for the susceptor and part to reach the processing temperature.
One existing control scheme employs thermocouples to provide feedback for power control about the thermal leveling point. The thermocouples are positioned in different locations about the work piece, and the temperature data from each of the thermocouples is used to calculate an average temperature. Each of the thermocouples must be properly calibrated so as to ensure accurate readings. In addition, the thermocouples are delicate and give faulty thermocouple readings when damaged. Such faulty thermocouple readings must be discovered and discarded before calculating the average temperature. Despite existing control schemes, improvements over the measurement and control of the temperature and timing of the induction heating process are still highly desired to produce parts of increasing quality.
Therefore, it would be advantageous to provide an induction heating system in which the temperature of the part can be easily controlled or fine-tuned. More particularly, it would be advantageous to have an induction heating control system that allows temperature control of a smart susceptor about its Curie point. Further, it would be advantageous to have an induction heating control system that did not require the use of multiple thermocouples, large amounts of data processing, or other complex electrical devices to monitor and control the temperature of the part.
The present invention addresses the above needs and achieves other advantages by providing an induction heating system for fabricating a part by heating and forming the part while more easily controlling the operating temperature. The induction heating system comprises a smart susceptor that includes a susceptor material that responds to an electromagnetic flux by generating heat and a cavity defined by the susceptor material that is configured to hold the part. An induction coil of the induction heating system is supplied with electrical power so as to generate the electromagnetic flux necessary for the susceptor to generate heat. A temperature controller includes a power supply that supplies electrical power to the induction coil. A controlling element of the temperature controller monitors trends in the electrical power supplied and changes the amount of electrical power being supplied so as to control the temperature of the part during fabrication.
In one embodiment, the present invention includes a smart susceptor, a coil and a temperature controller. The smart susceptor includes a susceptor material that defines a cavity that is configured to receive the part. The susceptor material is configured to respond to an electromagnetic flux by generating heat. Generation of heat by the susceptor material increases the temperature of the part in the cavity. The coil is positioned in proximity to the smart susceptor and is capable of generating the electromagnetic flux when supplied with electrical power. The temperature controller of the induction heating system has a power supply and a controlling element. The power supply is operably connected to the electromagnetic coil so as to supply an amount of the electrical power to the electromagnetic coil. The controlling element is configured to measure trends in the amount of the electrical power supplied by the power supply and is further configured to change the amount of the electrical power being supplied so as to control the temperature of the part in the cavity during fabrication.
The controlling element is further configured to continuously vary the amount of electrical power supplied to the coil in order to follow a predetermined pattern for the temperature of the part.
The susceptor material has a high magnetic permeability when below a Curie temperature and a low magnetic permeability when above the Curie temperature. Preferably, a predetermined maximum temperature necessary for fabrication of the part is approximately equal to the Curie temperature of the susceptor material. In such an aspect, the controller may be further configured to reduce the amount of electrical power supplied to the coil as the temperature of the susceptor material reaches the Curie temperature.
The temperature controller may include a voltage sensor operable to measure a voltage across the coil and wherein the controlling element may be further configured to control the amount of power supplied in response to a change in the voltage. In particular, the controller is configured to control the amount of power supplied to the coil so as to maintain a predetermined voltage measured by the voltage sensor.
The cavity may completely enclose the part. Optionally, the induction heating system further comprises a die having two portions and the smart susceptor has two separable portions. Each of the portions of the smart susceptor are attached to a respective one of the portions of the die. The die is configured to hold the portions together so as to define the cavity.
In another aspect, the coil defines a coolant pathway configured to receive a fluid coolant which draws heat from the coil during fabrication of the part.
The present invention has several advantages. Measurements of the voltage, or power, supplied to the coil provides an indication of when the susceptor temperature has reached the Curie point. Control of the power being supplied to the coil, therefore, allows the temperature of the susceptor, and hence the part, to be fine tuned without the use of complex electrical control devices and thermocouples that are prone to inaccuracy and breakage. Restated, the amount of power being supplied to the coil provides an indication of the global temperature of the part being manufactured and provides an effective indication of leveling off, or stabilization, of the part temperature. In addition, the power, or voltage, being supplied is a single number that can be easily monitored and controlled. Further, the susceptor requires no calibration beyond its initial chemical composition because there is no variation in the Curie point of the susceptor material.